Global parameter pre-optimization and nonlinear optimization for the Super Tau-Charm Facility

The Super Tau-Charm Facility (STCF) is a proposed high-luminosity dual-ring e + e - collider operating in the beam energy range of 1-3.5 GeV. To reach luminosities above 0.5 × 10 35 cm -2 s -1 at the optimal energy of 2 GeV, the STCF lattice adopts the crabwaist collision scheme, for which the preservation of dynamic aperture, momentum acceptance, and Touschek lifetime is a major challenge. This paper discusses selected nonlinear optimization issues for STCF as a representative low-energy crab-waist collider. A pre-optimization method is used to determine machine parameters consistent with luminosity, beam-beam limits, collective effects, and lifetime constraints. Particular attention is given to working-point selection, tune-footprint control, octupole compensation of final-doublet fringe-field detuning, and the interplay between lattice nonlinearities and beam-beam effects. The results show that beam-beam-aware nonlinear optimization and appropriate octupole configurations are important for improving the dynamic aperture and Touschek lifetime of STCF.